The invention relates to a control valve device which controls the flow of liquids by driving the valve piece while converting the rotary motion of a stepping motor to a linear motion.
A conventional control valve device using a stepping motor converts the rotary motion of the stepping motor into a linear motion using a conversion mechanism so that the valve piece can reciprocate linearly. A control valve device of this type will be described with reference to FIG. 1.
FIG. 1 is a sectional view showing a conventional control valve device. In FIG. 1, the control valve device is formed of a valve body 1 and a drive unit 2 that is secured to the valve body 1. The valve body 1 includes a piping path 1a for causing a control liquid to flow therethrough and a valve seat 1b confronting a valve piece (described later). The drive unit 2 includes: a stepping motor 4 for rotating a rotor shaft 3; a holder 5 for converting the rotary motion of the rotor shaft 3 into a linear motion; and the valve 6 secured to the front end of the holder 5.
The rotor shaft 3 of the stepping motor 4 is rotatably supported by a bearing 8 and a bearing 10. The bearing 8 is arranged on an inner peripheral portion of a housing 7 that is secured to the valve body 1, and the bearing 10 is arranged on an inner peripheral portion of a cover 9 that covers the rear side of the housing 7. To the outer periphery of the rotor shaft 3 is a multipolar magnet 11 firmly attached by a mold 12 so that the magnet is concentric with the rotor shaft 3. On the outer periphery of the magnet 11 is a stator 13. Reference numeral 14 designates a coil of the stator 13; 15, a mold for protecting the coil 14 and a coil terminal 16; and 17, an externally connecting terminal that is connected to the terminal 16. A packing 18 is interposed between the housing 7 and the cover 9. A spring 19 serves to prevent the rotor shaft 3 from backlashing in the axial direction and is resiliently interposed between the cover 9 and the bearing 10.
The holder 5 made of a synthetic resin has a substantially cylindrical form and is supported so as to be inserted into the rotor shaft 3 while causing a portion on the rotor shaft 3 projecting from the housing 7 to pass therethrough. The valve piece 6 is inserted and secured to an opening of the holder 5 on the front end side. The valve piece 6 is also unitarily formed of the synthetic resin. The base portion on the side of the projecting portion on the rotor shaft 3 is provided with a male screw, while the inner peripheral portion of the holder 5 is provided with a female screw that can be screwed into the male screw. A guide 20 serves to regulate the rotation of the holder 5 by the rotary motion of the rotor shaft 3. The guide 20 is secured to the housing 7. More specifically, when the rotor shaft 3 is rotated by the stepping motor 4, the holder 5 not only gets screwed into the rotor shaft 3 but also is then kept from rotating by the guide 20. As a result, the rotary motion of the holder gets converted into a linear motion, thereby causing the holder 5 to travel along the length of the rotor shaft 3. A spring 21 serves to prevent the holder 5 from slightly moving back and forth due to backlash of the screwed portion, and is interposed between the holder 5 and the guide 20 so that the holder 5 can be urged to the front side (toward the valve seat 1b).
A stopper 22 serves to regulate the maximum advance position of the holder 5 and is held by a stopper ring 23 at an extreme end of the rotor shaft 3. Reference numeral 24 designates a stopper for regulating the maximum retreat position of the holder 5.
The control valve device thus constructed causes the holder 5 that is screwed into the rotor shaft 3 to travel frontward and backward by rotation of the stepping motor 4. This causes the valve piece 6 to move back and forth to change the distance between and valve seat 1b and itself, i.e., the surface area of a fluid path, thereby controlling the flow rate of the fluid flowing through the pipe path 1a. The maximum advance position of the valve piece 6 is regulated by the holder 5 being abutted against the stopper 22, while the maximum retreat position thereof is regulated by the holder 5 being abutted against the stopper 24.
Thus, in the conventional control valve device as constructed above, the valve piece 6 is moved by a predetermined distance intermittently (or "stepwise") using the stepping motor 4 as its drive unit. As a result, there arises a problem when the valve is to be closed completely. Specifically, it is difficult to bring the valve piece 6 into intimate contact with the valve seat 1b when completely closing the valve that is moving stepwise; or, in some cases, the valve piece 6 is not brought into contact with the valve seat 1b, leaving a gap therebetween. In such a case, although in complete closure, the flow of the fluid in the valve piece 6 cannot be completely shut off. If the valve piece 6 is advanced until it comes into contact with the valve seat 1b to check leakage, the valve piece 6 is forcibly pressed into the valve seat 1b, which is a problem.